Variable frequency detector system



1942- i L; E. THOMPSON VARIABLE FREQUENCY DETECTOR SYSTEM Filed Oct. 31, 1940 Zhwentor Patented May 12, 1942 2,282,910. 7 I VARIABLE FREQUENCY DETECTOR SYSTEM Leland E. Thompson,

Merchantyille, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 31, 1940, SerialNo. 363,744

7 Claims. (crest-27) This invention relates to detector systems for frequency modulated and other variable frequency signals including a frequency discriminator network and a balanced rectifier output circuit from which audio frequency signals and control potentials resulting from frequency variation or signal modulation are taken.

It is an object of this invention to provide an improved variable frequency detector system which is automatically balanced for amplitude modulated signals and for amplitude modulation on a desired frequency modulated signal,

It is a further object of the present invention to provide a frequency discriminator network having an improved balanced output circuit for maintaining a resistance or voltage balance therein eiiective to prevent the transfer therethrough of output voltages resulting from amplitude modulated signals or amplitude variations of a frequency modulated signal or carrier wave.

The invention wilLhowever, be better understood from the following description, when considered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

Referring to the drawing, the figure is e a schematic circuit diagram of a variable frequency detector system embodying the invention, and having as a signal source the output amplifier stage of an intermediate frequency am plifier. The system of the present example is coupled to the amplifier output circuit 6 by means of two primary windings 1 and 8, in series and A circuit of this type is effective to prevent the transmission of amplitude modulated signals and amplitude variations of a frequency modulated signalgp rovided that the center or carrier frequency' of a received signal does not vary or drift and provided that the tuning of the circuits'l3 and i4 does not vary or drift. However, as is well known, both the center frequency of a recoupled to corresponding secondary windings 9 and It, which are tuned to resonance above and below a predetermined center frequency by suitable tuning capacitors indicated at H and I2.

The two tuned frequency discriminator circuits I3 and I4 thus provided are connected to balanced rectifier means comprising a tube I5 having an anode l6 connected with the high potential terminal I! of the circuit I3 and having a second anode l8 connected with the high potential terminal IQ of the circuit M.

The low potential terminals of the tuned circuits Hi and I4 are connected to a common low potential lead 26 which, in turn, is connected to the center tap 2| of a balanced output resistor or impedance networkcomprising two series sections or branch circuits 22 and 23, the outer terminals 24 and 25 of which are connected, respectively, with cathodes 26 and 2-1 in the rectifier l5, associated withthe anodes I6 and 18, respectively.

This circuit arrangement provides one wellwhich the output potentials vice or tube, the value of by'suitable control electrode means.

ceived signal and the tuning of the signal circuits may change for various reasons, such as temperature change in certain of the circuit components, both at the signal source and at the receiver. 7

In accordance with the invention, this tendency' to drift is compensated in the output network of the detector system to maintain a resistanceor a voltage balance effective to prevent transmission of amplitude modulated signals and amplitude variations of a variable frequency signal therethrough.

'Accordingly, the two output branches 22 and 23 of the balanced rectifier I5 are provided with suitable shunt tuning capacitorsv 30 and 3| by at the terminals 24 and 25 are brought into opposite phase relation one with respect to the other, and a portion of each of the branches is shunted by the anodecathode impedance of an electric discharge de- In the present example, a dual amplifier tube 35 is provided with a tube element comprising a cathode 36, a control grid 37 and an anode 38 connected respectively to the center terminal 2|, a first intermediate terminal 39 and a second intermediate terminal 40, the latter being more adjacent to the outer terminal 24.

Likewise, a second tube element is provided comprising a cathode 4| connected, with thecathode 36, to the center terminal 2|, a control grid 42 connected to anrintermediate terminal 43 corresponding or in'balanced relation to the terminal 39 on theopposite side of the tap 2|, and an anode 44 connected to a terminal 45 more adjacent to the outer terminal 25 and corresponding or in balanced relation to the terminal 40 on the opposite side of the center tap 2|.

The control grid connection in each-caseincludes a filter resistor 46 and filter capacitor 41 providing a-predetermined time constant for the which is controllable 1 application of potentials to the control grids and 55 connected between the terminals and v 2|. They may, however, comprise any suitable impedance elements or may be provided as a sin- V gle unit having taps thereon as shown? The circuit arrangement is such. that a porput resistor network is shunted by the anodecathode impedance of a tube element of the tube 35. In the present example, the anode-cathode impedance 3638 is connected in shunt across the resistor sections 5| and '52 between the terminals 40 and 2|. Likewise, the anode-cathode impedance 4|-44 is connected across the resistor sections 54 and 55 between the terminals 45 and 2|.

The signal output from the balanced network is taken from suitable points in balanced relation to each other on opposite sides of the center tap 2| and preferably as shown, from the anode terminal 45 through a lead 6|] which is preferably grounded as indicated at 6|, and a second output lead 62 connected with a suitable movable tap connection 63 adjacent to the terminal 40.

The output circuit is connected to an amplifier circuit such as an audio frequency amplifier stage 64 comprising an amplifier tube 65 havin a control grid 66 coupled through a capacitor 61 with the lead 62 and having a cathode 68 connected through a bias resistor 69 with the output lead 60, Thesignal output from the amplifier stage is taken through the anode circuit indicated at 10.

If a received variable frequency or frequency modulated carrier wave or the circuit tuning of the receiving system should drift in frequency, the circuit shown operates to maintain a voltage balance of the undesired amplitude modulation resulting therefrom, the amplitude modulation being either noise voltages or an interfering heterodyne signal.

The operation of this circuit is as follows: The load circuit of the rectifier |'6--26 comprises the resistor sections 50, 5| and 52, together with the balancing capacitor and the anode-cathode impedance of the balance control tube section 36-38. The load circuit of the rectifier |82'| comprises the resistor sections 53, 54 and 55, together with the balancing capacitor 3| and the anode-cathode impedance of the balance control tube section 4|-44. The capacitors 30 and 3| are adjusted to obtain the same phase relationship between current and voltage in the two balanced diode circuits or branches 22 and 23.

If the signal carrier current is the same through each branch, any amplitude modulation voltage between the output terminals 63 and 2| and between the output terminals 2| and 45 may be adjusted by means of the contact 63 to be substantially equal and out of phase, so that these voltages cancel and do not appear on the output circuit between the terminals 63 and 45, and hence are not amplified through the output stage 64.

If the frequency of the signal carrier changes .15 tion of the impedance of each branch of the outor the receiver circuits drift out of exact tuning, then the current through branch 22, for example, may be assumed to increase while that through the branch 23, under the same assumption, will decrease. connected at points 39 and 43, with signal rectification both grids will assume a positive potential, with respect to the cathodes, which will increase on the grid 31 and decrease on the grid 42 under the condition of unbalance.

The anode circuit resistance of the control tube section between the terminals 40 and 2| will decrease, while the anode circuit resistance between the terminals 45 and 2| will increase under the..conditions above assumed thereby causing i a portion of the increased current flow in the branch 22 to be shunted through the tube elements 38-36, and restoration of the current flow in the branch 23 of the output network resulting from a decrease in the current flow between the tube elements 44-4|, until a condition of equilibrium is established at which the grids 31 and 42 are at substantially the same positive potential, thus maintaining the voltage between terminals 63 and 2| and between the terminals 2| and 45 substantially the same as it was before the carrier frequency or the circuit tuning drifted from the normal value.

Thus it will be seen that a balance is maintained between the output terminals 63 and 45 with respect to the common terminal 2| to a direct current produced by a signal carrier. If this current is modulated in amplitude, the modulation will be out of phase and the signal will not pass to the audio frequency amplifier stage 64. However, if the signal is modulated by a frequency change, the discriminator circuit produces a change in current through the branch 22 which is out of phase with the current change which occurs in the branch 23 so that the resultant audio frequency output voltage passes to the audio frequency amplifier 64.

Thus the frequency discriminator network provides a balanced output circuit for maintaining a resistance or voltage balance therein effective to prevent the transfer therethrough of output voltages resulting from amplitude modulated signals or amplitude variations of a frequency modulated signal or carrier wave.

I claim as my invention:

1. The combination with a frequency modulated signal conveying circuit, of a frequency discriminator network coupled thereto, balanced signal rectifier means connected with said network, a balanced output circuit for said rectifier means including two balanced impedance branch circuits having a common center terminal connection, means providing the anode-cathode impedance of a controlling electric discharge device across a portion of each of said branch circuits in balanced relation, grid control means for varying said anode-cathode impedances having balanced control connections on each of said branch circuits, and means providing a balanced output circuit connection with each of said branch circuits.

2. The combination with a frequency modulated signal conveying circuit, of a frequency discriminator network coupled thereto, balanced signal rectifier means connected with said network, a balanced output circuit for said rectifier means including two impedance branch circuits having a common center terminal, variable shunt capacity means for balancing said branch circuits, means providing the anode-cathode im- Since the control grids are' pedance of an electric discharge device across a portion of each of said branch circuits in balanced relation with respect to said center terminal, grid control means for varying said anode-cathode impedances in opposition in re.- sponse to frequency drift and amplitude modulation signals, having balanced bias potential supply connections on each of said branch circuits, and means providing a balanced output circuit connection with each of said branch circuits.

3. A variable frequency detector system comprising, in combination, a frequency discriminator network and rectifier means in balanced relation thereto for the detection of variable frequency signals, means including a series of impedance elements having a common center output terminal and two end terminals providing a balanced output circuit for said rectifier means, an electric discharge device having the anodecathode impedance thereof connected between said common terminal and an intermediate tap on either side thereof in connection with said series connected impedance elements, a control grid for each of said anode-cathode impedances connected on either side of said center terminal, and means providing a balanced signal output connection with said series connected impedance elements on either side of said center terminal.

4. A variable frequency detector system'comprising, in combination, a frequency discriminator network and balanced rectifier means for the detection of variable frequency signals, means providing a balanced output impedance network.

means comprising two series for said rectifier branch circuits having a common center tap and two outer terminals, electric discharge tube elements for controlling the impedance of each of said branch circuits having cathodes connected with said center tap and having anode elements connected at points on said branch circuits intermediate between said center and outer terminals to provide the anode-cathode impedance of said tube elements in balanced relation across a portion of each of said branch circuits, a control grid for each of said means providing a control grid connection from each of said grids to the respective branch circuit across which the anode-cathode impedance controlled thereby is connected at a point intermediate between the anode connection therefor and the common center terminal.

5. A variable frequency detector system comprising, in combination, a frequency discriminator network, rectifier means connected with said network in balanced relation thereto, means providing a balanced output circuit for said last tube elements, and

named rectifier means including a series of impedance elements connected on either side of a common output terminal, means providing balanced output circuit connections including certain of said impedance elements on "opposite sides of said common output terminal, electric discharge control means having cathode elements connected with said common output terminal and having anode elements connected in balanced relation to each other on opposite sides thereof with said series connected impedance elements, and means providing control grid connections for said electric discharge control means on opposite sides of said common output terminal for maintaining a resistance and voltage balance in said output circuit to'fprevent the transfer therethrough of output voltages resulting from amplitude modulated signals and amplitude variations of a frequency modulated signal in said network.

6. A variable frequency detector system comprising, in combination, means for amplifying received variable frequency signal energy, means for detecting said signal energy comprising detecting circuits connected in balanced opposition to amplitude modulated signals andin cut-ofphase relation to variable frequency signals, a balanced output circuit for ergy, means in said circuit for maintaining a predetermined phase relation between current and voltage in the detected signal energy in said output circuit, and means for changing the impedance of the branches circuit in response to amplitude variations in the substantially constant.

7. In a detector for variable frequency signals, means for eliminating amplitude modulated signals and disturbances which comprises two detector circuits connected in opposition to amplitude modulated signals and inout-of-phase relation to variable frequency signals, an input circuit for applying received signals to said detector circuits, a balanced output circuit for said detectors including impedance elements for maintaining thesame phase relation between current and voltagein the outputs of said circuits, and a variable impedance element in shunt relation to at least a portion of each branch of said balanced output circuit responsive to the carrier voltage input to said detector circuits for changing the impedance in said output circuit branches to maintain the signal amplitude equal therein.

LELAND E. THOMPSON.

detected signal en-.

substantially 

